Olive oil, a dietary staple in Mediterranean regions, is now a healthy favorite oil in the US, valued not only for its flavor but also its health benefits.

Rich in monounsaturated fats, olive oil may help lower your risk of heart disease and may even benefit insulin levels and blood sugar control, helping to lower the risk of type 2 diabetes.

As with most foods, however, not all olive oil is created equal. There's a wide variation between high-quality and low-quality oils, and even among the best varieties, rancidity is a major problem.

4 Signs of Defective Olive Oil

When you spend the money on a quality bottle of olive oil, you want to know that you're getting what you pay for. However, there are many factors that influence quality, from how long the olives sat before processing to how long you've left the oil sit out on your counter.

As The Olive Oil Times reported, paying attention to these four potential defects can help you weed out the good oils from the bad:

1. Rancidity

Olive oil is highly perishable, but is generally said to be 'good' for two years from the date it was bottled (this will usually be the 'Best By' date). However, a better indicator of freshness is to go by its harvest date, which will tell you when the oil was actually made. Only select oils that have this information on the bottle.

So the first step is finding an oil that was harvested as recently as possible. From there, many other factors, including storage temperature, exposure to air and light, the level of antioxidants and chlorophyll content in the oil, will also influence how resistant it is to going rancid.

All olive oil will get rancid eventually, but if you're like most people, you're probably leaving your bottle of olive oil right on the counter, opening and closing it multiple times a week (or even a day). Every time the oil is exposed to air and/or light, it undergoes oxidiation and will get rancid quicker.

Extra-virgin olive oil, in particular, also contains chlorophyll that accelerates decomposition and makes the oil go rancid even faster than semi-refined olive oils, according to oil expert Dr. Rudi Moerck. So how can you tell if your olive oil is rancid?

It smells like crayons or putty

It tastes like rancid nuts

It has a greasy mouthfeel

Unfortunately, as The Olive Oil Times reported:

"The sad truth is that most people in the US… are accustomed to the flavor of rancid olive oil."

2. Fusty Oil

'Fusty' oil occurs when olives sit too long (even just a few days) before they're milled, leading to fermentation in the absence of oxygen. Fusty flavors are incredibly common in olive oil, so many simply think it's normal. However, your olive oil should not have a fermented smell to it, reminiscent of sweaty socks or swampy vegetation.

"A good way to taste an example of the fusty defect involves table olives," The Olive Oil Times reported. "Look through a batch of Kalamata-style olives and see if you can find any that are not purple or maroon-black and firm, but instead are brown and mushy. Eat one. THAT is the flavor of fusty."

3. Moldy Olives

If your olive oil tastes dusty or musty, it's probably because it was made from moldy olives, another occasional olive oil defect.

4. Wine or Vinegar Flavor

If your olive oil tastes like it has undertones of wine and vinegar (or even nail polish), it's probably because the olives underwent fermentation with oxygen, leading to this sharp, undesirable flavor.

Extra Virgin Olive Oil Is One of the Most Commonly Adulterated Foods

The four defects above are examples of what commonly occurs due to poor processing methods or handling. However, olive oil is also a common target of food fraud, in which it is deliberately adulterated at your expense, according to the U.S. Pharmacopeial Convention's (USP) Food Fraud Database. Even "extra virgin" olive oil is often diluted with other less expensive oils, including hazelnut, soybean, corn, sunflower, palm, sesame, grape seed and walnut. But these other oils will not be listed on the label, nor will most people be able to discern that their olive oil is not pure.

If you live in an area where olive oil is made, buying from a local producer is the ideal solution as it allows you to know exactly what's in your oil. If not, try an independent olive oil shop that can tell you about the growers, or at least seek out a brand name that you trust to produce quality oil from your local supermarket.

If at all possible, taste the oil before you buy it. While this won't necessarily be a guarantee of quality (especially if you're not skilled at picking out all the potentially subtle taste differences), it can help you to pick out the freshest-tasting oil possible (and if you open a bottle at home and find that it tastes rancid or 'bad,' return it to the store for a refund).

The Fridge Test: Not a Good Measure of Olive Oil Quality

Earlier this year, The Dr. Oz Show featured a segment on the olive oil 'fridge test,' which suggested that you can tell your extra virgin olive oil is pure if it solidifies in the fridge. The US Davis Olive Center decided to test the theory out and found that this is actually a very unreliable way to detect olive oil purity.

In fact, the Olive Center researchers refrigerated seven samples of oil and found that none of them congealed after 60 hours in the fridge. While some had areas that had hardened, due to the varying levels of saturated fats in the oil, none solidified completely. So you can save yourself the effort and avoid using this test.

"All olive oils contain a small amount of saturated fatty acids that solidify at refrigerator temperatures," said Paul Vossen, UC Cooperative Extension advisor. "The amount of solidification is equal to the amount of saturated fatty acids in the oil, which depends mostly on the varieties of olives used to make the oil and to a lesser extent where the olives were grown. Solidification does not indicate freshness, purity, flavor, extra virgin grade, or any other quality parameter."

Are You Cooking With Olive Oil? Stop!

Olive oil is an ideal oil when it's used cold, such as drizzled over a salad or on top of homemade hummus. However, it's important to realize olive oil is NOT good for cooking. Due to its chemical structure and a large amount of monounsaturated fats such as oleic acid, cooking makes extra virgin olive oil very susceptible to oxidative damage.

Consuming oxidized, rancid oil is not going to benefit your health, so when you need an oil to cook with, coconut oil is the ideal choice, because it is one of the only commonly used vegetable fats stable enough to resist heat-induced damage. Remember, olive oil is excellent when used for cold dishes, but cooking with it is virtually guaranteed to damage this highly heat-sensitive oil.

Keep in a cool, dark place -- dark is key because light will most definitely oxidize the fats in olive oil

Purchase smaller bottles rather than larger to ensure freshness

Immediately replace the cap after each pour

To further help protect extra virgin olive oil from oxidation, Dr. Moerck suggests putting one drop of astaxanthin into the bottle. You can purchase astaxanthin, which is an extremely potent antioxidant, in soft gel capsules. Just prick it with a pin and squeeze the capsule into the oil. The beautiful thing about using astaxanthin instead of another antioxidant, such as vitamin E, is that it is naturally red, whereas vitamin E is colorless, so you can tell the oil still has astaxanthin in it by its color. As the olive oil starts to pale in color, you know it's time to throw it away.

Generally speaking, olive oil is best consumed within a year of harvest, although most will last for up to two years from harvest when unopened and kept in a cool dark place. Oils that have a more bitter, peppery flavor have a higher polyphenol content, and these oils will generally keep better than oils made from ripe olives, which have a softer flavor. The latter should be used within six months to a year at most.

This is yet another reason to purchase olive oil in small bottles, rather than large, as it is easier to use up in a shorter period. If you purchase a large amount of olive oil you may be tempted to keep it even though it has gone rancid.

Disclaimer: The entire contents of this website are based upon the opinions of Dr. Mercola, unless otherwise noted. Individual articles are based upon the opinions of the respective author, who retains copyright as marked. The information on this website is not intended to replace a one-on-one relationship with a qualified health care professional and is not intended as medical advice. It is intended as a sharing of knowledge and information from the research and experience of Dr. Mercola and his community. Dr. Mercola encourages you to make your own health care decisions based upon your research and in partnership with a qualified health care professional. If you are pregnant, nursing, taking medication, or have a medical condition, consult your health care professional before using products based on this content.

If you want to use an article on your site please click here. This content may be copied in full, with copyright, contact, creation and information intact, without specific permission, when used only in a not-for-profit format. If any other use is desired, permission in writing from Dr. Mercola is required.

Disclaimer: The entire contents of this website are based upon the opinions of Dr. Mercola, unless otherwise noted. Individual articles are based upon the opinions of the respective author, who retains copyright as marked. The information on this website is not intended to replace a one-on-one relationship with a qualified health care professional and is not intended as medical advice. It is intended as a sharing of knowledge and information from the research and experience of Dr. Mercola and his community. Dr. Mercola encourages you to make your own health care decisions based upon your research and in partnership with a qualified health care professional. If you are pregnant, nursing, taking medication, or have a medical condition, consult your health care professional before using products based on this content.

If you want to use an article on your site please click here. This content may be copied in full, with copyright, contact, creation and information intact, without specific permission, when used only in a not-for-profit format. If any other use is desired, permission in writing from Dr. Mercola is required.